Skeletogenesis in the limbs, trunk and other sites involves endochondral and intramembranous ossifications. While much is known about these processes at the embryological, histological and biochemical levels, relatively little is known at the molecular level. In Preliminary Studies, we describe the cloning of avian C-1-1, an alternatively spliced form of the ets transcription factor ERG-3. We find that C-1-1 is expressed in permanent articular chondrocytes while ERG-3 is expressed in prehypertrophic growth plate chondrocytes in the developing chick skeleton. Virally driven constitutive expression of C-1-1 blocks chondrocyte maturation and endochondral ossification, while constitutive expression of ERG-3 favors maturation. These and other results lead to our central hypothesis that C-1-1 and ERG-3 are involved and have distinct roles in skeletogenesis and skeletal function. Specifically, we propose (a) to determine whether constitutive expression of C-1-1 or ERG-3 affects skeletogenesis differentially by changing the expression of potential downstream effectors, including Indian hedgehog, PTH-RP, PTH-RP receptor or the bone factor CBFA1, (b) to determine the developmental consequences of ERG inhibition caused by antisense strategies or gene ablation obtained by homologous recombination in mice, and (c) to determine how C-1-1 and ERG-3 differentially regulate gene transcription in chondrocytes and whether they may do so by specific interactions with ets DNA motifs and other transcription factors such as ETS-2 and AP-1 complexes. To achieve these goals, we will use viral expression vectors, cell cultures, in vivo chick embryo manipulation, recombinant proteins, protein-protein and protein-DNA interaction assays, in situ hybridization and gene ablation. The results of the project will provide key information on molecular mechanisms regulating the formation of articular cartilage, long bones and other elements, and will point to molecular defects that may underlie congenital and acquired conditions of skeletogenesis and skeletal function in the growing organism and adult.